#include <bits/stdc++.h>
using namespace std;
int main() {

  int n;
  cin >> n;
  vector<int> a(n + 1);

  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  long long total = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= n; j++) {
      total += a[i] * a[j];
    }
  }
  cout << total << endl;

  return 0;
}